Various polymeric materials have been widely used over the past 70 years for molding various jars, caps, tool handles, toothbrushes and other consumer products. In many of these applications the molded part must be clear, tough, impact resistant, as well as having a pleasing appearance. In order to obtain a clear part it is desirable that the polymer have a slow crystallization rate. The occurrence of crystallinity results in the formation of haze in the part. A slower crystallization rate also reduces the chance for orientation induced crystallization in the gate area. Thus, there is the potential for improved gate appearance. In addition it is desirable to use a polymeric material with good flow (low melt viscosity) to insure the ease of completely filling the part. Furthermore, it is desirable to be able to make the parts with a short cycle time in the injection molding process to maximize the efficiency with which parts can be made. One of the main factors controlling the cycle time of a polymeric material is the time it takes to cool below its glass transition temperature during the injection molding process. Thus a glass transition temperature near the injection molding process temperature is desirable to obtain a reduced cycle time.
Polyester materials such as poly(ethylene terephthalate) (PET) and poly(1,4-cyclohexylenedimethylene Terephthalate) (PCT) have many desirable properties for molded parts but these polymers crystallize readily which results in the formation of hazy parts especially in the case of thick walled parts or during a two shot molding process.
It is possible to reduce the rate of crystallization by incorporation of additional monomers into these polyesters. However, it is important to choose the composition of these copolyesters such that not only is the crystallization rate decreased but all of the other desirable features (like cooling efficiency, flow ability, resin toughness) of a molding composition discussed above are obtained.
The present invention describes copolyesters which have a very slow crystallization rate while also giving all of the other desired attributes discussed above.